1. Field of the Invention
The present invention relates to the field of power station technology. It refers to a method for regulating the power of a turbo set converting thermal power into electric power, said turbo set comprising, on a common shaft, a turbine driven by the thermal power and a generator driven by the turbine and delivering electric power to a network.
2. Discussion of Background
The conventional way of regulating the output power of a gas turbine is to measure the electric output power PG delivered at the output terminals of the associated generator, to compare the measured value with a predetermined power value (desired value) PC and to transmit the resulting differential signal .DELTA.P=PG-PC as a control signal to a power regulator which regulates the thermal power of the gas turbine.
An illustrative circuit diagram for such known power regulation is reproduced in FIG. 1. The power of a turbo set 10 consisting of a gas turbine 15 and of a generator 16 is regulated. The gas turbine 15 comprises the actual turbine 11, a combustion chamber 12, a compressor 13 and a controllable inlet 14 for the combustion air, said inlet usually consisting of adjustable inlet guide vanes (Variable Inlet Guiding Vanes VIGVs). The turbine 11 and generator 16 are seated on a common shaft 17, the rotary frequency f of which is measured by means of a rotary frequency encoder 25. The generator 16 delivers the generated electric power PG to a network 18, as a rule a three phase network. The electric power PG of the generator 16 is compared, in a subtractor 19, with a predetermined power value PC and the difference .DELTA.P is fed to a power regulator 20 which in turn, via the controllable inlet 14, controls the quantity of combustion air fed to the compressor 13 and the mass flow dmfc/dt of the fuel fed to the combustion chamber 12.
The predetermined power value (desired value) PC is obtained from the sum PCt=PC*+.DELTA.PCt of a reference power value PC* and a correction value .DELTA.PCt. The correction value .DELTA.PCt, for its part, derives from a characteristic encoder 23 which, in the event of a deviation of the measured rotary frequency f from a rotary frequency desired value fc, outputs an appropriate correction value in accordance with the difference .DELTA.f formed in a subtractor 24 and with a predetermined characteristic .DELTA.PC=K(.DELTA.f). A rate limiter 21 is additionally provided, which limits the rate of change of the regulating signal.
It has now emerged, that the regulating circuit illustrated in FIG. 1 may lead to a possibly hazardous behavior of the gas turbine when high (positive or negative) accelerations of the shaft occur. In this case, the measured (electric) output power PG of the turbo set is no longer a measure of the generated thermal power PT of the gas turbine 15, said thermal power being determined by the mass flows of the combustion air and fuel, but additionally contains an appreciable fraction of kinetic power. The resulting inequality between the measured output power and the generated thermal power may cause power regulation to initiate changes (unjustified per se) in the mass flows of combustion air and fuel which may be hazardous to the gas turbine itself and/or to the stability of the connected network.
Furthermore, if the generator switch opens, the measured electric power PG at the generator falls to zero, since there is no flow of power into the network. In this case, too, the result is inequality between the measured output power and the generated thermal power, and power regulation receives false information on the thermal state of the gas turbine, thus leading to an undesirable behavior of the power regulator.
The fundamental cause of the problems mentioned becomes clear when the following power equation for the rotor of the gas turbine is set up: EQU (1) PG=PT-Pkin,
PT signifying the effective thermal output power of the gas turbine, and EQU (2) Pkin=4.pi..sup.2.theta.f(df/dt)
being the kinetic power of the shaft assembly, with the moment of inertia .theta. of the shaft assembly the, rotary frequency f of the shaft assembly and the rotary frequency change (acceleration) df/dt of the shaft assembly. The shaft assembly refers to the shaft as well as all rotating masses fixed to the shaft, for example rotor disks, blades, and so forth. It becomes clear from the equations (1) and (2) that the measurement of the electric output power PG is, in general, not directly a measure of the thermal power at the gas turbine, but a measure of the total power at the shaft, and includes the kinetic power which is delivered and consumed respectively in the event of braking and acceleration of the shaft.
This results in the following undesirable power regulation behavior patterns:
1. Thermal relief during a delivery of kinetic power
In this case, the rotor (shaft assembly) is sharply braked. This takes place, typically, when the gas turbine is synchronized with a network which undergoes a sharp fall in frequency. As a result of this fall, the rotor delivers a large amount of kinetic power, thus leading to a sudden rise in the measured output power PG. If the desired value PC does not change appreciably, power regulation reduces the thermal power PT, in order to keep the measured output power PG as near as possible to the desired value PC. However, this is precisely the wrong response of the regulating system, because a frequency fall in the network is a sign of an increased power requirement. Instead, gas turbines having sufficient power reserve should increase the thermal power, instead of reducing it, in order to assist in stabilizing the network. In addition, due to the delivery of kinetic power, the thermal relief of the gas turbine may lead to flame extinguishing which aggravates even further the already existing power deficit of the network. This behavior therefore, altogether, puts network stability at serious risk.
2. Thermal charging during load shedding
What is considered here, is the situation of a gas turbine which runs synchronously with a stable network at a constant speed and with a specific power (for example, 160 MW thermal power). Since the speed of the shaft is constant (df/dt=0; Pkin=0), the total measured electric power PG is, according to equation (1), identical to the thermal power PT. When the generator switch opens, then, the measured electric output power PG falls to zero. The power regulator consequently receives a signal representing an output power of 0 MW, even though the thermal power is actually unchanged (in the example, 160 MW). The power regulator is thereby falsely induced to increase the thermal power by an amount which is demanded by the power desired value. Theoretically, as a result, the thermal power may increase to double the power desired value PC. As soon as the generator switch opens, the rotor is accelerated by the thermal power PT. The fall of the signal PG and the resulting increased thermal power of the gas turbine will further increase the acceleration of the shaft, so that the shaft possibly reaches a speed limit range.